1. Technical Field
This invention relates to the technology of thermally spraying metallic coatings onto metal substrates and more particularly to coatings that have both enhanced machinability and enhanced wear resistance when adhered to light weight metals, such as aluminum.
2. Discussion of the Prior Art
It is known to apply thermally sprayed coatings to cylinder bores of a cast aluminum engine block to improve the wear resistance of the aluminum surface and thereby eliminate (a) the need for inserting cast-iron liners to withstand the sliding contact of steel piston rings, or (b) the need to use high silicon content aluminum alloys that require treatment to precipitate hard wear particles to withstand sliding contact. The thermally sprayed coatings can be applied in a relatively thin thickness, i.e. 100-1000 micrometers (0.1-1.0 mm. or 0.0004-0.04 inches), to promote economy of materials while meeting performance needs.
One of the drawbacks to using thermally sprayed coatings on cylinder bores is that it has prompted a significant change in the method of finishing the coated surface of the bores due in part to their increased surface hardness. Such increased hardness leads to shorter tool life during machining. Honing tools are generally self-aligned and floatingly supported to follow the geometry of the coating on the cylinder bore, rather than to remove material about an axis independent of the coated surface. Honing of this type is undesirable because thermally sprayed coatings tend to vary in thickness from side to side as well as from top to bottom of the bore wall. Such honing procedure may produce uneven coating removal and variations in coating thicknesses along the cylinder bore wall. If the finished surface is not in absolute alignment with the axis of the crank bore, proper fit of the piston assembly may be difficult or lead to premature wear. It would be desirable and cost effective if the thermally sprayed coating were to be deposited in a manner to allow for direct honing or machining of the confine that is oriented to the true axis of the crank bore (bearing housing) without the fear that the coating will be distorted or injured.